1. Field of the Invention
The present invention relates to a sheet-supplying device employed in an image-forming device.
2. Description of the Related Art
Sheet-supplying devices using air suction are well known in the art as sheet-supplying devices used in printers, photocopiers, and other image-forming devices. FIG. 1 illustrates the function of a sheet-supplying device 101 of this type. As shown in FIG. 1, the sheet-supplying device 101 includes a sheet-accommodating tray 111 and a sheet-feeding device 112 disposed above the sheet-accommodating tray 111. The sheet-feeding device 112 includes a sheet-feeding belt 112C and an air suction device 113. A plurality of sheets 102 is stacked on top of the sheet-accommodating tray 111. The sheet 102 positioned on the top of the stack is referred to as a topmost sheet 102A. When the air suction device 113 draws the topmost sheet 102A up to the sheet-feeding belt 112C, the sheet-feeding belt 112C conveys the topmost sheet 102A to a prescribed position.
An air-jetting device 114 is disposed on the front side of the sheet-accommodating tray 111, that is, on the left side in FIG. 1. The air-jetting device 114 blows a jet of air onto one end of the sheets 102, causing a few sheets 102 stacked on the sheet-accommodating tray 111 to float and separate. A position sensor 116 is disposed on the other end of the sheets 102, which end is unaffected by the air blown from the air-jetting device 114 and does not float. The position sensor 116 includes a digital sensor 116A and a surface position-detecting lever 116B. A raising/lowering device (not shown) is connected to the sheet-accommodating tray 111 and is capable of raising and lowering the same. The raising/lowering device raises the sheet-accommodating tray 111 until the topmost sheet 102A contacts the surface position-detecting lever 116B. At this time, the surface position-detecting lever 116B is displaced vertically upward, enabling the position sensor 116 to detect the position of other end of the topmost sheet 102A based on the position of the surface position-detecting lever 116B.
A plurality of suction holes (not shown) is formed over the entire surface of the sheet-feeding belt 112C. By means of these suction holes and the air suction device 113, sheets can be drawn to and conveyed by the sheet-feeding belt 112C.
Next, steps in a control process of the controlling device (not shown) for controlling operations of this conventional sheet-supplying device will be described with reference to the flowchart in FIG. 2. A controller (not shown) monitors whether the position sensor 116 has detected that the topmost sheet 102A has contacted the surface position-detecting lever 116B (S101). If the position sensor 116 has not detected this contact (S101: NO), the controller raises the sheet-accommodating tray 111 (S102), and then the controller monitors again whether the position sensor 116 has detected this contact (S101). When the position sensor 116 detects that the topmost sheet 102A has contacted the surface position-detecting lever 116B (S101: YES), then the controller judges whether the sheet-accommodating tray 111 has stopped (S103).
When the sheet-accommodating tray 111 has not stopped (S103: NO), the controller stops the raising of the sheet-accommodating tray 111 (S104). Then, the controller controls the air-jetting device 114 to blow air onto the one end of the sheets 102 stacked on the sheet-accommodating tray 111 and the air suction device 113 to draw air (S105).
The air blown from the air-jetting device 114 floats and separates a few of the top sheets 102. The topmost sheet 102A that is among the separated sheets 102 is drawn to the sheet-feeding belt 112C by the air suction device 113. The topmost sheet 102A that has been drawn to the sheet-feeding belt 112C is conveyed to a prescribed position by the rotation of the sheet-feeding belt 112C. When the topmost sheet 102A has been conveyed in this way, the controller repeats S101-S105 with respect to the following topmost sheet 102A.
By the way, when the number of sheets 102 stacked on the sheet-accommodating tray 111 decreases and the vertical position of the topmost sheet 102A becomes lower. Then, the topmost sheet 102A cannot be detected by the position sensor 116 (S101: NO). In such a case, the controller raises the sheet-accommodating tray 111 (S102). On the other hand, when the number of sheets 102 has not likely decreased by much and the vertical position of the topmost sheet 102A has not dropped that much, the topmost sheet 102A is detected soon by the position sensor 116 (S101: YES).
Japanese unexamined patent application publication No. HEI-7-187422 discloses a sheet-supplying device that can adjust the position and angle of the jetted air based on the vertical position of the topmost sheet. Accordingly, this sheet-supplying device can separate floating sheets from one another, even when the sheets have a tendency to curl.
Japanese unexamined patent application publication No. HEI-7-89625 discloses a sheet-supplying device 201 as shown in FIG. 3 that includes an air-jetting device 214 for blowing a jet of air, and a reflecting type distance-measuring sensor 215 for measuring part of a topmost sheet 202A that is floated by air blown from the air-jetting device 214. With this construction, the sheet-supplying device 201 can adjust the amount of jetted air from the air-jetting device 214 based on the vertical position of the topmost sheet 202A measured by the reflecting type distance-measuring sensor 215.
However, when there is a curl in the sheets, the vertical position of the topmost sheet differs between its front end and rear end, that is, between the left and right ends in FIG. 1. Since the conventional sheet-supplying devices as shown in FIG. 1 use a position sensor to detect only the position of the sheet at the rear end thereof, the front end of the topmost sheet may not be in an optimal position for being drawn to and conveyed by the sheet-feeding device, even if the vertical position of the topmost sheet at the rear end thereof is in the optimal position. This may result in such problems as a plurality of sheets being fed simultaneously in the sheet-feeding operation or the feeding belt being unable to draw the topmost sheet by air suction and therefore failing to feed the sheet.
Further, although the sheet-supplying device disclosed in Japanese unexamined patent application publication No. HEI-7-187422 can adjust the position and angle of the jetted air based on the vertical position of the topmost sheet, this construction is complex and increases the manufacturing costs of the device. Further, since the optimal vertical position of the topmost sheet for pickup by air suction differs based on the sheet weight and quality, feeding failures and the feeding of multiple sheets simultaneously may occur when different types of sheets are used.
The sheet-supplying device disclosed in Japanese unexamined patent application publication No. HEI-7-89625 uses a reflective type distance-measuring sensor to measure the vertical position of the topmost sheet on the end of the sheet that is floating and fluttering by the jetted air. Such measurements are unreliable and do not produce accurate values. Therefore, this device cannot reliably output an optimal volume of air.